New calculations reveal van der Waals dispersion contributions of H₂-H system

A better understanding of the dipole moment induced by H₂-H interactions is important in determining the absorption and emission of radiation during collisions of dihydrogen molecules and hydrogen atoms — a phenomenon that plays a pivotal role in our ability to estimate the age of the universe based on the rate of heat loss from white dwarf stars.

Lee et al. report finite-field calculations at the unrestricted coupled-cluster level for the dipole induced by interactions between H₂ and H. By including more orientation angles, bond lengths and H₂-H separations than ever before, the group identified subtle effects that depend on the full electronic charge distribution.

“While a hydrogen molecule is colliding with a hydrogen atom, the entire system acquires a dipole moment,” said Katharine Hunt, an author of the paper. “This induced dipole moment permits transitions between vibrational and rotational states that would be forbidden for isolated hydrogen molecules.”

For the first time, the researchers obtained the van der Waals dispersion contributions to the dipole directly from calculations on H₂-H. In particular, the dispersion dipole poses acute numerical challenges, since it varies as the inverse seventh power of the intermolecular separation at long range. Their results also indicate the distortions of the electronic charge distribution, which give rise to the dipole, to account for the attractive van der Waals forces between all molecules.

The researchers plan to next explore higher temperatures and shorter intermolecular distances than in current calculations, eventually permitting new astrophysical applications.

Source: “The interaction-induced dipole of H₂-H: New ab initio results and spherical tensor analysis,” by Hua-Kuang Lee, Xiaoping Li, Evangelos Miliordos, and Katharine L. C. Hunt, The Journal of Chemical Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5098900 .